[libstdc++] Fix test timeout in stop_calback/destroy.cc

A change was made to __atomic_semaphore::_S_do_try_acquire() to
(ideally) let the compare_exchange reload the value of __old rather than
always reloading it twice. This causes _M_acquire to spin indefinitely
if the value of __old is already 0.

libstdc++-v3/ChangeLog:
	* include/bits/semaphore_base.h: Always reload __old in
	__atomic_semaphore::_S_do_try_acquire().
	* testsuite/30_threads/stop_token/stop_callback/destroy.cc:
	re-enable testcase.